The deposition of solids onto heat transfer surfaces of steam generating equipment, such as industrial boiler equipment, is a major problem. Common contaminants in boiler feedwater that can form deposits are calcium and magnesium salts (hardness), carbonate salts, sulfate, phosphate, siliceous matter, and iron oxides. Any foreign matter introduced into the boiler in soluble or particulate form will tend to form deposits within the boiler and to a great extent on the heat transfer surfaces. Formation of deposits on the heat transfer surfaces will decrease the efficiency under which the heat transfer takes place, and can lead to overheating, circulation restrictions, damage to the systems, loss of effectiveness, and increased costs due to cleaning, unscheduled outages, and replacement of equipment. In an extreme case, catastrophic tube failure can occur.
Polymeric deposit control agents are frequently added to the feedwaters of boilers. Their ultimate objective is to inhibit the formation of deposits on the heat transfer surfaces and to facilitate the removal of any deposits in the blowdown and prevent deposition within the boiler system. This is accomplished via two mechanisms: a solubilization mechanism, where chelants, or chelant-type molecules, form soluble complexes with the deposit forming species which are removed in the blowdown; and an adsorption mechanism where the deposit control agent is adsorbed on the surface of the particulate matter and inhibits the formation and crystal growth of the depositing species, and disperses the deposit that is being formed, and makes it more readily removable.
At the high operating pressures and temperatures of steam generating systems, polymeric dispersants must not only contain effective chemistry to inhibit deposit formation but also must demonstrate sufficient thermal stability to remain effective. Under steam generating conditions, all polymeric materials experience some degree of thermal degradation that is dependent on structure, pressure/temperature, medium composition, and residence time within the boiler. Many polymeric materials typically employed in these applications remain stable and effective at pressures up to 300 psig but can begin to experience severe degradation as pressures are increased further, e.g., to 300 psig and above. Further, at the high temperatures and pressures in such systems, this decrease in efficacy can necessitate increased polymer feed levels and high system treatment costs. The thermal degradation in some cases can result in increased heat transfer deposition and organic fouling. Therefore, there exists a need for polymeric dispersants that are thermally stable and efficacious at pressures ranging up to and above 900 psig.